Cylinder/Head/ intake Gaskets/ exhaust gaskets? I know there is a few intake gaskets per carb to go on either side of cooling shroud. I personally would not use starting fluid as it removes any lubrication. Maybe use a squirt bottle and inject some mixed gas through carbs to see if it is not getting fuel?
I put new gaskets on around the shroud and I took the gas line off the carburetor pulled it over a couple times and gas came out put the gas line back on and all I could get was a little puff of smoke out of the intake and the backfire once in awhile?
Did you have to split the cases to change the crank seals? I used to have a TX 340 and the crank seals were changed without splitting the case. I had issues once where I didn’t lube( grease the inside of seals , on rebuild) it ran on one cyl though but flooded the other
Seal leakage can do some weird things. Make sure they fit snug on the outside where they join the case. Maybe some loctite could be put there?they usually fit tight on the case but liberally grease where they contact the crank and the innner lip